The long-term objective is to discover the function(s) of sleep. Rats chronically sleep deprived by brief, forced locomotion at sleep onset have shown an increased thermoregulatory setpoint (leading to an early increase in body temperature), excessive heat loss (leading to a later decline in body temperature), and hormonal changes and increased metabolism appropriate to compensatory thermogenesis. Rats died after an average of 2.5 weeks of sleep deprivation. Yoked control rats which received the same stimulation, but randomly with respect to sleep, showed only minor effects. The aims of the proposed research are as follows. 1. To determine whether the deaths of sleep deprived rats are caused by their excessive heat loss, their body temperatures will be incubator-regulated to near normal. Survival times and sleep deprivation symptoms will be compared to those of sleep deprived rats maintained at a constant, "normal" ambient temperature. 2. To determine whether self-controlled body temperatures prolong survival during sleep deprivation, rats will be provided with operant control of ambient temperature. Body temperature, survival time, and symptoms of sleep deprivation will be compared with those of yoked controls and sleep deprived rats without operant temperature control. 3. To determine whether increased metabolism or other effects of hyperthyroidism increase or decrease survival and other symptoms, thyroxine will be chronically administered to sleep deprived rats. Survival times and sleep deprivation symptoms will be compared with those of yoked hyperthyroid control rats and euthyroid sleep deprived rats. 4. To determine whether the increased thermoregulatory setpoint of sleep deprived rats is mediated by prostaglandins and/or opioids, they will be given aspirin or naltrexone to determine whether either reverses the increase in body temperature which occurs early in sleep deprivation. 5. To determine whether sleep deprivation causes a desensitization or downregulation of central adrenoreceptors, regional brain adrenoreceptor number and affinity will be compared in sleep deprived rats, yoked control rats, and normally maintained rats. 6. To evaluate whether a need for paradoxical (REM) sleep is associated with low thermal resistance, rat strains with high and low amounts of paradoxical sleep will be compared on thermal resistance. Elucidation of the function of sleep could have important implications for human disorders of insomnia and hypersomnia, for efficient and healthy sleep hygiene, and for defining sleep requirements.